MOS half-bridge drive circuit, particularly for power MOS half-bridges

ABSTRACT

A drive circuit includes a voltage source supplying a reference voltage at its output; a voltage elevating circuit connected to a supply voltage and to the output of the voltage source, and supplying at its output, under normal operating conditions, a drive voltage greater than the supply voltage and increasing with the reference voltage. The input of the voltage source is connected to the output of the voltage elevating circuit, and defines a positive feedback path resulting in an increase in the reference voltage corresponding to an increase in the drive voltage, and therefore results in a corresponding increase in the drive voltage up to a maximum permissible value, thus providing for a sufficient drive voltage for driving the gate-source junction of power MOS transistors, even in the presence of a low supply voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a MOS half-bridge drive circuit,particularly for power MOS half-bridges.

2. Discussion of the Related Art

Power MOS half-bridges generally include a pair of power MOStransistors, usually N-channel type, push-pull connected as shown by wayof example in FIG. 1. FIG. 1 shows a half-bridge 1 including two MOS(typically VDMOS-Vertical Double Diffused MOS) transistors M₁ and M₂.Transistor M₁ (top transistor) has its drain terminal connected tosupply voltage VS, and its source terminal connected to the drainterminal of transistor M₂ (bottom transistor), the source terminal ofwhich is grounded. The gate terminals of transistors M₁ and M₂ areconnected respectively to top and bottom drivers T and B which, in turn,are connected to a synchronizing input signal IN, for producing drivesignals. One of transistors M₁, M₂ is turned off when the other is on.

The output of half-bridge 1 thus presents a signal VOU_(T), theamplitude of which switches between R_(DSon2) * I (where R_(DSon2) isthe equivalent resistance between the drain and source of transistor M₂when saturated, and I is the output current) and V_(s) -R_(DSon1) * I(where R_(DSon1) is the equivalent resistance between the drain andsource of transistor M₁ when saturated).

In half-bridge 1, to achieve a low voltage drop in whichever oftransistors M₁, M₂ is on at the time, and therefore a high excursion ofoutput voltage V_(OUT), the transistor must be driven by a highgate-source voltage (VGS), typically about 10 V, which provides foroptimizing saturation resistance R_(DSon).

Such a high voltage is usually achieved without difficulty with respectto transistor M₂, which permits the use of a reference voltageobtainable from the supply voltage. The situation is more complex,however, in the case of transistor M₁, which requires a drive voltage of10 V+V_(OUT), i.e. 10 V+V_(S) -R_(DSon1) * I, which is higher than thesupply voltage.

A highly effective conventional solution to the problem is to use acharge-pump or voltage doubling circuit, which gives a voltage higherthan the supply voltage, and provides for driving one or morehalf-bridges. A simplified diagram of such a circuit, indicated byreference character 2, is shown in FIG. 2.

Charge-pep circuit 2 includes an oscillator 3; two condensers 4 and 5;and two diodes 6 and 7. Oscillator 3 is connected to a reference voltageV_(REF) and ground, so that its output 8 switches continually betweenV_(REF) and 0 V. In the first half-cycle, when output 8 is 0 V,condenser 4 charges to V_(S) via diode 6; whereas, in the secondhalf-cycle, when output 8 equals V_(REF), condenser 4 (whose terminal 9,not connected to oscillator 3, switches to voltage V_(REF) +V_(S) inrelation to ground) transfers the charge to condenser 5. In the secondhalf-cycle, diode 6 disconnects condensers 4 and 5 from the supplyvoltage V_(S), while, in the first half-cycle, in which condenser 4restores the charge transferred to condenser 5, diode 7 disconnectscondenser 5 from condenser 4 and the supply voltage V_(S).

Under normal operating conditions, if V_(CP) is the output voltage ofcircuit 2 (drive voltage), and V_(be) the voltage drop in diodes 6 and7:

    V.sub.CP =V.sub.S -2 i V.sub.be +V.sub.REF                 ( 1)

and, assuming in this particular case that V_(be) =0.7V and V_(REF) =10V:

    V.sub.CP =V.sub.S -1.4V+10 V≈V.sub.S +9.6V

Charge-pump circuit 2 therefore provides a drive voltage ensuring asufficient voltage drop V_(GS) in transistor M₁ to effectively saturateit.

Reference voltage V_(REF) is generally produced using circuit 10 in FIG.3, which presents a minimum voltage difference D_(V) between supply andreference voltages V_(S) and V_(REF). Circuit 10 includes a bipolar NPNtransistor 11; a resistor 12 connected between the base and collector oftransistor 11; and two diodes and 14, of which diode 13 is a Zenerdiode, connected in series between the base of transistor 11 and ground.The emitter of transistor 11 is connected to supply voltage V_(S), andthe base defines the output of circuit 10.

If R, in circuit 10, is the resistance of resistor 12; h_(fe) thecurrent gain for small signals of transistor 11; and I_(O) the emittercurrent of transistor 11; when, as in this case, V_(S) <V_(Z) +V_(be)(where V_(Z) and V_(be) are the voltage drop in Zener diode 13 and diode14 respectively):

    D.sub.V =V.sub.S -V.sub.REF =(R*I.sub.O)/h.sub.fe +V.sub.be

which, by appropriate sizing of the components, may be reduced to 1 V,so that:

    V.sub.REF =V.sub.S -D.sub.V =1V                            (2)

There are some situations, however, in which the circuit shown in FIG. 3is impractical, as in the case of automotive applications, which requirethat the MOS half-bridge, operating normally with a supply voltage V_(S)of 6 V, should also be capable of operating with a minimum supplyvoltage V_(Smin) of 5.4 V, i.e. that the drive circuit of top transistorM₁ supply an output voltage V_(CP) of at least roughly 9.3 V.

In fact, to achieve a fairly good saturation resistance, V_(DMOS)transistors must be driven in such a manner as to present a voltage dropV_(GS) of at least 4 V. This is obtained easily, in the case oftransistor M₂, even with a minimum supply voltage V_(Smin). The samedoes not apply, however, to transistor M₁, which, with a minimum supplyvoltage V_(Smin) of 5.4 V, requires that V_(OUT) ≈ 5.3 V and, therefore,V_(CP) ≈ 9.3 V.

According to equation (2), however, the circuit shown in FIG. 3 circuitprovides, at most, supplying an output voltage V_(REF) of 4.4 V, sothat, according to equation (1), charge-pump circuit 2 provides solelyfor supplying a voltage V_(CP) of:

    V.sub.CP =5.4 V-1.4 V+4.4 V=8.4 V

The source-drain voltage drop of transistor M₁ therefore equals:

    V.sub.GS1 =V.sub.CP -V.sub.OUT =3.1 V

which is well below the required value.

Another known solution for generating the reference voltage V_(REF)includes using a PNP transistor in place of transistor 11, as shown inFIG. 4, where PNP transistor 15 has an emitter connected to supplyvoltage V_(S), and a collector defining the output of the circuit. Aresistor 16 is connected parallel to the base-emitter junction oftransistor 15; a current source 17 is connected between the base oftransistor 15 and ground; and a Zener diode 18 is connected between theoutput and ground.

The above solution, however, despite providing for a voltage drop D_(V)=V_(S) -V_(REF) equal to the emitter-collector saturation voltage oftransistor 15 (and, therefore, below that of the circuit shown in FIG.3), is not used for the following reasons:

a) the maximum output current I_(OUT) required by the load must besupplied at all times by transistor 15, even when not needed (thusresulting in high power consumption and dissipation); and

b) for a given area, a PNP transistor provides for carrying a muchsmaller current than an NPN type, and the Zener diode must be sized towithstand all the current (thus increasing the size of the integratedcircuit).

It is, therefore, an object of the present invention to provide a MOShalf-bridge drive circuit enabling the half-bridges to operate even witha low supply voltage.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a MOS half-bridgedrive circuit, particularly for power MOS half-bridges, including afirst reference voltage line set to a first reference voltage. A voltagegenerating circuit, coupled to the first reference voltage line, has anoutput including a second reference voltage line, and supplies a secondreference voltage at its output. A voltage elevating circuit has firstand second inputs coupled respectively to the first reference voltageline and the output of the voltage generating circuit, and has anoutput. The voltage elevating circuit supplies at its output a drivevoltage greater than the first reference voltage and increasing with anincrease in the second reference voltage. The voltage elevating circuitincludes a feedback circuit increasing the second reference voltage to amaximum normal operating value in response to an increase in the drivevoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred, non-limiting embodiment of the present invention will bedescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1 shows a simplified diagram of a conventional MOS half-bridge forwhich the drive circuit according to the present invention is designed;

FIG. 2 shows a simplified electric diagram of a known charge-pumpcircuit;

FIG. 3 shows an electric diagram of a known reference voltage source;

FIG. 4 shows an electric diagram of a further known reference voltagesource;

FIG. 5 shows a simplified electric diagram of the drive circuitaccording to the present invention; and

FIG. 6 shows an alternate embodiment electric diagram according to thepresent invention.

No further description of FIGS. 1-4, which have already been describedabove, will be given herein after, and any component parts of the knowncircuits (in particular, the charge-pump in FIG. 2) forming part of thedrive circuit, according to the present invention, will be indicatedusing the same reference characters.

Drive circuit 20 shown in FIG. 5 includes a linear voltage regulator 21;a firing circuit 22; a charge-pump circuit 23; a comparator stage 24; aload 25; a first group of bottom drivers B; and a second group of topdrivers T.

Linear voltage regulator 21 (defining a voltage source) includes anoperational amplifier 30, the output 31 of which is connected to thegate terminal of a preferably V_(DMOS) transistor 32, the drain terminalof which is connected to supply line 29 (reference potential line onwhich voltage V_(S) is present), and the source terminal of which isgrounded via two series connected feedback resistors 33 and 34. The midpoint between the source terminal of transistor 32 and resistor 33defines output 35 of regulator 21 at which reference voltage V_(REF) ispresent; while the mid point between feedback resistors 33 and 34 isconnected to the inverting terminal of operational amplifier 30. Thenon-inverting terminal of operational amplifier 30 receives a precise,temperature-stable voltage V_(bg) produced, for example, by a so-calledband-gap circuit (not shown). Operational amplifier 30 also includes asupply input 36 connected to supply line 29. Regulator 21 also includesa current mirror circuit 37 comprising two P-channel MOS transistors 38and 39. The gate and drain terminals of transistor 38 are shortcircuitedand connected to a current source 40; while the drain terminal oftransistor 39 is connected to output 31 of operational amplifier 30. Thesource terminals of both transistors 38 and 39 are connected to output42 of charge-pump circuit 23 via line 43.

Firing circuit 22 includes a bipolar NPN transistor 44 having itscollector connected directly to supply line 29, its base connected tosupply line 29 via a resistor 45, and its emitter connected to output 35of regulator 21 and to input 46 of oscillator 3 of charge-pump circuit23. As in the circuit shown in FIG. 2, charge-pump circuit 23 (whichforms a voltage elevating circuit) also includes a condenser (capacitor)4 having one terminal connected to the output of oscillator 3, and theother terminal connected to the cathode of diode 6, the anode of whichis connected to supply line 29. The cathode of diode 6 is also connectedto the anode of diode 7, the cathode of which defines output 42 to whichdrive voltage V_(CP) is supplied. Condenser 5 is disposed between output42 and ground.

Comparator stage 24 (forming a switch mechanism for turning off firingcircuit 22) in turn includes a comparator 48, the non-inverting input ofwhich is connected to output 42 of charge-pump circuit 23, and theinverting input of which receives reference voltage V_(TR). The outputof comparator 48 is connected to the gate terminal of a MOS transistor49, the source terminal of which is grounded, and the drain terminal ofwhich is connected to the base of transistor 44 of firing circuit 22.

Load 25 and bottom drivers B are connected to output 35 of regulator 21,while top drivers T are connected to output 42 of charge-pump circuit23. Bottom drivers B drive respective bottom transistors M₂.1 -M₂.N ofhalf-bridges 1.1-1.N, by switching them according to the operatingfrequency of the half-bridges, receive respective synchronizing inputsignals I_(N).1 -I_(N).N, which are also supplied to respective topdrivers T for driving top transistors M₁.1 -M₁.N of the samehalf-bridges 1.1-1.N in the same way as, but in phase opposition todrivers B, and with a higher voltage, as already described. Top andbottom drivers T and B are also grounded, while the respectivehalf-bridges 1.1-1.N provide outputs OUT₁ -OUT_(N).

The FIG. 5 drive circuit operates as follows.

With a supply voltage V_(S) of 5.4 V on line 29, the firing circuit isdesigned to give a voltage drop V₁ of 1 V, so that V_(REF) =4.4 V.

In the above condition, oscillator 46 of charge-pump circuit 23 comesinto operation, and, on the basis of equation (1), charge-pump circuit23 supplies an operating output voltage of:

    V.sub.CP =V.sub.S +V.sub.REF -2 V.sub.be =5.4+4.4-1.4=8.4 V.

The above voltage V_(CP) is supplied to current mirror circuit 37, whichthen drives transistor 32 by bringing its gate terminal up to voltageV_(CP). Transistor 32 therefore provides a voltage drop V_(GS)(overdrive) of:

    V.sub.GS =8.4-4.4=4 V.

This is sufficient to saturate transistor 32, so that voltage V_(REF)begins to rise, thus resulting, on the basis of equation (1), in acorresponding increase in voltage V_(CP), and a further increase involtage V_(REF). A positive reaction is thus initiated, which continuesuntil transistor 32 is fully saturated and (assuming a 100 mVdrain-source saturation voltage V_(DSsat) of transistor 32) referencevoltage V_(REF) reaches the maximum value

    V.sub.REF =5.4 V-100 mV=5.3 V

and

    V.sub.CP =5.4+5.3-1.4=9.3 V.

Prior to reaching the normal operating condition, the reduction in thedifference between V_(S) and V_(REF) is also accompanied by a reductionin the voltage drop at the base-emitter junction of transistor 44 offiring circuit 22, until transistor 44 eventually goes off.

Under normal operating conditions, top and bottom drivers T and Bfunction correctly, by virtue of the gate-source junction being suppliedwith a voltage V_(GS) of at least 4 V.

In actuality, the positive reaction, and therefore operation of thedrivers and respective half-bridges, is also initiated at lower supplyvoltages (V_(S) =4 V), even though the equivalent drain-sourcesaturation resistance is less than optimum.

Firing circuit 22 is also turned off by comparator stage 24, especiallyin the presence of a high supply voltage V_(S) (which may be as high as60 V), and, in particular, when the output voltage V_(CP) of charge-pumpcircuit 23 is such as to ensure the system is turned on, and prior tooutput voltage V_(REF) of regulator 21 reaching the normal operatingvalue. In fact, as soon as drive voltage V_(CP) exceeds voltage V_(TR)at the inverting input of comparator 48, the output of comparator 48switches from low to high, thus turning on transistor 49, and thevoltage at the drain terminal of transistor 49 becomes low, thusinversely biasing the base-emitter junction of transistor 44.

Consequently, even in the presence of a high supply voltage V_(S),output voltage V_(REF) of regulator 21, on reaching the set value,maintains this value by virtue of being independent of supply voltageV_(S). A further advantage of actively turning off firing circuit 22 (byconnecting the base terminal of transistor 44 to a voltage close to 0 V)is that it enables transistor 44 to withstand a breakdown voltageBV_(CBO).

As supply voltage V_(S) increases beyond the value at which transistor32 is no longer saturated, operational amplifier 30, which, with a lowsupply voltage V_(S), is incapable of supplying V_(REF), provides aprecise, temperature-stable, regulated reference output voltage:

    V.sub.REF =V.sub.bg (1+R.sub.2 /R.sub.1)

where R₂ and R₁ are the respective resistances of resistors 33 and 34.By appropriate sizing, it is therefore possible to obtain an optimumvoltage V_(REF) of 10 V.

The advantages of the circuit shown in FIG. 5 will be clear from theforegoing description. In addition to providing for turning on anddriving one or more MOS half-bridges, even in the presence of a lowsupply voltage of 5.4 V, the circuit according to the present inventionalso provides for generating an optimum reference voltage, e.g. of 10 V,as required for driving MOS half-bridges in the best possible manner.The circuit is also straightforward in design, and is easilyimplementable, for example, as an integrated circuit.

To those skilled in the art it will be clear that changes may be made tothe circuit as described and illustrated herein without, however,departing from the scope of the present invention. In particular, shouldfiring circuit 22 be insufficient for supplying the current required, itmay be replaced by circuit 22' shown in FIG. 6.

In place of one transistor 44, circuit 22' features two transistors: aPNP transistor 53 with its emitter terminal connected to supply line 29,its base terminal connected to one terminal of resistor 45 and to thedrain terminal of transistor 49 of comparator stage 24, and itscollector terminal connected to the base terminal of a second NPNtransistor 54, the collector terminal of which is connected to supplyline 29, and the emitter terminal of which is connected to output 35 ofregulator 21. Finally, a current source 55, supplying current I₁, isprovided between the base terminal of transistor 53 and ground.

The FIG. 6 circuit also provides a voltage drop V₁ =V_(S) -V_(REF) =1 V,but, in comparison with circuit 22, a much higher output current 12equal to:

    I.sub.2 =β.sub.1 ·β.sub.2 ·I.sub.1

where β₁ and β₂ are the current gain of transistors 53 and 54respectively.

We claim:
 1. A MOS half-bridge drive circuit comprising:a firstreference voltage line set to a first reference voltage; a secondreference voltage line; a voltage generating circuit, coupled to thefirst reference voltage line, having an output coupled to the secondreference voltage line, supplying a second reference voltage at itsoutput; and a voltage elevating circuit having a first and second inputcoupled respectively to the first reference voltage line and the outputof the voltage generating circuit, and having an output, and supplyingat its output a drive voltage greater than the first reference voltageand increasing with an increase in the second reference voltage; whereinthe voltage elevating circuit includes a feedback circuit increasing thesecond reference voltage to a maximum normal operating value in responseto an increase in the drive voltage.
 2. A drive circuit as claimed inclaim 1 wherein the voltage generating circuit further includes avoltage regulator having an input coupled to the output of the voltageelevating circuit.
 3. A drive circuit as claimed in claim 2 wherein thevoltage regulator includes an output transistor having a controlterminal coupled to the output of the voltage elevating circuit, and anoutput terminal, the output of the voltage generating circuit beingprovided on the output terminal.
 4. A drive circuit as claimed in claim3 wherein the output transistor includes a MOS transistor having a gateterminal coupled to the output of the voltage elevating circuit, asource terminal including the output terminal of the voltage generatingcircuit, and a drain terminal connected to the first reference voltageline.
 5. A drive circuit as claimed in claim 4 further comprising:athird reference voltage line receiving a third reference voltage; and aresistive feedback; wherein the voltage regulator includes anoperational amplifier having a non-inverting input coupled to the thirdreference voltage line, an inverting input coupled to the outputterminal of the voltage generating circuit through the resistivefeedback circuit, and an output coupled to the control terminal of theoutput transistor.
 6. A drive circuit as claimed in claim 5 furthercomprising a current mirror circuit coupled to the control terminal ofthe output transistor and the output of the voltage elevating circuit.7. A drive circuit as claimed in claim 6 further comprising a firingcircuit coupled to the first reference voltage line and the second inputof the voltage elevating circuit.
 8. A drive circuit as claimed in claim7 wherein the firing circuit includes a transistor having a firstterminal coupled to the first reference voltage line, a second controlterminal coupled to the first reference voltage line, and a thirdterminal coupled to the second input of the voltage elevating circuit.9. A drive circuit as claimed in claim 8 wherein the transistor of thefiring circuit includes a bipolar transistor.
 10. A drive circuit asclaimed in claim 8 wherein the transistor of the firing circuit includestwo cascade-connected bipolar transistors.
 11. A drive circuit asclaimed in claim 10 further comprising a switch coupled to the controlterminal of the transistor of the firing circuit, turning off the firingcircuit prior to the drive circuit reaching a normal operatingcondition.
 12. A drive circuit as claimed in claim 11 wherein the switchcircuit includes a comparator stage having a first input coupled to theoutput of the voltage elevating circuit, a second input coupled to athreshold potential, and an output coupled to the control terminal ofthe transistor of the firing circuit.
 13. A drive circuit as claimed inclaim 1 wherein the voltage elevating circuit includes a charge-pumpcircuit.
 14. A drive circuit as claimed in claim 13 wherein thecharge-pump circuit includes:first and second capacitors, the firstcapacitor having a first terminal-and a second terminal; first andsecond diodes; and an oscillator stage coupled to the output of thevoltage generating circuit, and an output coupled to the first terminalof the first capacitor, the second terminal of the first capacitorcoupled to the first reference voltage line through the first diode andcoupled to the output of the voltage elevating circuit through thesecond diode, the second capacitor coupled to the output of the voltageelevating circuit and a third reference voltage line.
 15. A MOShalf-bridge drive circuit comprising:a first reference voltage line setto a first reference voltage; a second reference voltage line; a voltagegenerating circuit, couples to the first reference voltage line and thesecond reference voltage line, supplying a second reference voltage onthe second reference voltage line; and a voltage elevating circuithaving a first input coupled to the first reference voltage line and asecond input coupled to the second reference voltage line and an output,and supplying at its output a drive voltage greater than the firstreference voltage and increasing with an increase in the secondreference voltage, the voltage elevating circuit includes a feedbackpath increasing the second reference voltage to a maximum normaloperating value in response to an increase in the drive voltage.
 16. Adrive circuit as claimed in claim 15 wherein the voltage generatingcircuit further includes a voltage regulator having an input coupled tothe output of the voltage elevating circuit.
 17. A drive circuit asclaimed in claim 16 wherein the voltage regulator includes an outputtransistor having a control terminal coupled to the output of thevoltage elevating circuit, and an output terminal including the outputof the voltage generating circuit.
 18. A drive circuit as claimed inclaim 17 wherein the output transistor includes a MOS transistor havinga gate terminal coupled to the output of the voltage elevating circuit,a source terminal including the output terminal of the voltagegenerating circuit, and a drain terminal connected to the firstreference voltage line.
 19. A drive circuit as claimed in claim 18further comprising:a third reference voltage line receiving a thirdreference voltage; and a resistive feedback circuit; the voltageregulator includes an operational amplifier having a non-inverting inputcoupled to the third reference voltage line, an inverting input coupledto the output terminal of the voltage generating circuits through theresistive feedback circuit, and an output coupled to the controlterminal of the output transistor.
 20. A drive circuit as claimed inclaim 19 further comprising a current mirror circuit coupled to thecontrol terminal of the output circuit and the output of the voltageelevating circuit.
 21. A drive circuit as claimed in claim 20 furthercomprising a firing circuit coupled to the first reference voltage lineand the second input of the voltage elevating circuit.
 22. A drivecircuit as claimed in claim 21 wherein the firing circuit includes atransistor having a first terminal coupled to the first referencevoltage line, a second control terminal coupled to the first referencevoltage line, and a third terminal coupled to the second input of thevoltage elevating circuit.
 23. A drive circuit as claimed in claim 22wherein the transistor of the firing circuit includes a bipolartransistor.
 24. A drive circuit as claimed in claim 23 wherein thetransistor of the firing circuit includes two cascade-connected bipolartransistors.
 25. A drive circuit as claimed in claim 24 furthercomprising a switch coupled to the control terminal of the transistor ofthe firing circuit, turning off the firing circuit prior to the drivecircuit reaching a normal operating condition.
 26. A drive circuit asclaimed in claim 25 wherein the switch circuit includes a comparatorstage having a first input coupled to the output of the voltageelevating circuit, a second input coupled to a threshold potential, andan output coupled to the control terminal of the transistor of thefiring circuit.
 27. A drive circuit as claimed in any one of claims17-26 wherein the voltage elevating circuit includes a charge-pumpcircuit.
 28. A drive circuit as claimed in claim 27 wherein thecharge-pump circuit includes:first and second capacitors, the firstcapacitor having a first terminal and a second terminal; first andsecond diodes; and an oscillator stage coupled to the output of thevoltage generating circuit, and an output coupled to the first terminalof the first capacitor, the second terminal of the first capacitorcoupled to the first reference voltage line through the first diode andcoupled to the output of the voltage elevating circuit through thesecond diode, the second capacitor coupled to the output of the voltageelevating circuit and a fourth reference voltage line.
 29. A MOShalf-bridge drive circuit comprising:first means for receiving a firstreference voltage; means, coupled to the first means for receiving, forsupplying a second reference voltage; and second means for receiving thesecond reference voltage; means, coupled to the first and second meansfor receiving, for supplying a drive voltage greater than the firstreference voltage that increases with an increase in the secondreference voltage; means for supplying feedback including means forcoupling the drive voltage the means for supplying the second referencevoltage.
 30. A method for driving a MOS half-bridge circuit comprisingthe steps of:supplying a first reference voltage line with a firstreference voltage; supplying, in response to the first referencevoltage, a second reference voltage line with a second referencevoltage; generating, in response to the first and second referencevoltages, a drive voltage greater than the first reference voltage thatincreases with an increase in the second reference voltage; andincreasing the second reference voltage to a maximum normal operatingvalue in response to an increase in the drive voltage.